Posting this for future searches:

Found one of the old manuals that talks about the rear springs being there for the reason I suspected - the cab sheet metal will rack or "oil can" due to frame flex. Can't remember who it was but someone had pictures of their frame work and saw they were adding lots of stiffening braces and boxing the frame channels- in this case going to a direct mount is probably fine.

'69 1/2T stepside, Sea Foam Green, 250 L6 w/ 3 on the tree, not an option on it, in the family since new.